Force, Motion and State of Motion


Force is the cause of change in the state of motion of a body or an object. It is a quantitative description of an interaction that causes a change in an object's motion. Force can cause an object to move or accelerate, to slow down or decelerate, to stop or to change its direction. The applied force can be a push, a pull or dragging of an object. 

Examples of Force in Everyday Life

  1. Applying brakes to stop a vehicle

  2. Lifting a load

  3. Pushing and pulling a door 

  4. Kneading and rolling the dough 

  5. Kicking a football 

  6. Stretching a spring or a rubber band 

  7. Attracting paper bits with an electrostatically charged comb

  8. Force exerted by our muscles while moving the limbs 

  9. Throwing a stone in the air and it’s coming down.

  10. A magnet attracting iron nails

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The State of Motion and Causes of Change in Motion? 

Motion is the change in position of a body with respect to its surrounding environment, within a given interval of time. An object is said to be in motion if its position changes with time, with reference to a fixed frame.

A chair cannot move its own, what do you do to make a chair move?

We often say that a force has been applied to the chair when it is pushed.

The motion of an object is explained by its speed and direction of motion. If an object is at rest, the state is considered to be in the state of zero. By applying force, we can change the position of the object or can say that the object is in motion. 

While taking a penalty kick in football, before being hit, the ball was at rest. Then, its speed was zero. The player applied the force on the ball. This applied force sets the ball in motion towards the goal.

Suppose, the ball hits the goal or the goalkeeper dives and saves the goal. In both conditions, the speed of the ball changes. Force can also cause an increase or decrease in motion if it is applied in the same direction or the opposite direction respectively.

Many times, an applied force may not result in any change in the state of motion.

What will happen when you push a wall with the maximum force that you can exert? 

No effect of force is observed.

How can Force Change the State of Motion?

  1. The Applied Force can Cause Acceleration

The change in motion is equivalent to a change in velocity. A change in velocity applies that there will be an acceleration. The force causes a change in motion. So it produces acceleration too.

If an object is stationary in the beginning, it accelerates when it starts to move. Likewise, if an object is already moving and a force is applied in the same direction, the object will accelerate as long as the force is applied to the object. If the force is removed, the acceleration will also stop.

For example, James was walking towards the north at a speed of 10 metres per second. James speeds up and now begins running towards the west at 20 metres per second after 5 seconds. In this case, James has accelerated his velocity by 2 m/s² i.e his velocity has increased by 2m/s every second. 

Another example is of an apple falling down. It starts falling at zero metres per second. At the end of the first second, the apple is moving at 9.8 metres per second. The apple has accelerated. This acceleration here is caused by gravity.

  1. The Applied Force can Cause Deceleration

If an object is moving and a force applied to it in the opposite direction of the motion, the object will decelerate or slow down.

Suppose, a cricketer hits the ball high up. It will slow down as it travels upwards due to the force of gravity. Likewise, a boat decelerates due to wind flowing opposite to the direction of motion of the boat.

Decelerating force can put a moving body to rest.

For example, when a car driver applies brakes, it begins to decelerate.

  1. Force can Cause a Change in the Direction of Motion

A change in either the speed of a moving body or its direction or both is referred to as a change in its state of motion. Thus, the force can change the direction of motion.     

For example- In a cricket match, a bowler bowls the ball towards the batsman with some velocity(u). The batsman hits the ball and it travels in a different path with another velocity(v). This is because the batsman applies force on the ball and changes the direction of the ball.

FAQ (Frequently Asked Questions)

Question 1: What is Meant by Constant Acceleration?

Solution: Let us take an example to understand this. 

Suppose, a car's velocity is changing over the course of time. Its velocity changes constantly by 10 m/s in each second of time. This acceleration is referred to as constant acceleration.

If an object is changing its velocity either by a constant amount or a varying amount, it is said to be an accelerating object. But, an object with a constant velocity (both speed and direction not changing) is not accelerating.

Since, the velocity is changing by a fixed amount each second, the object is said to be in a state of constant acceleration.

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Question 2: Explain the Relationship Between Force and Acceleration.

Solution: The velocity of an object changes when an external force acts on it. For an object with a constant mass m, the force F is the product of the object's mass and its acceleration a:

F = m × a

When a force is applied externally, the change in velocity depends on the mass of the object. An applied force results in a change in velocity; and similarly, a change in velocity will generate a force. The equation works both ways. This equation has been derived from Newton's Second Law of Motion.